In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium, to form characters and image. The ink-jet printing methods have been rapidly spread because of their various advantages such as easiness of full coloration, low costs, capability of using plain paper as the recording medium, non-contact with printed images and characters, etc.
As to the ink-jet printing methods, for example, WO 01/096483 discloses an ink for ink-jet printing which contains a colorant formed by coating a pigment and/or a dye with a polymer, water, and at least one compound selected from the group consisting of an acetylene glycol-based surfactant, an acetylene alcohol-based surfactant, a glycol ether and a 1,2-alkylene glycol, and exhibits excellent dispersion stability and ejection stability, and capable of providing images which are free from bleeding and excellent in color developability and rubbing resistance.
JP 2003-096345A discloses a pigment-containing ink for ink-jet printing which is composed of a polymer emulsion containing polymer fine particles into which a water-insoluble or hardly-soluble colorant is included, and which also contains at least one wetting agent selected from the group consisting of glycerol, 1,3-butanediol, triethylene glycol, 1,6-hexanediol, propylene glycol, 1,5-pentanediol, diethylene glycol, dipropylene glycol, trimethylol propane and trimethylol ethane, and further contains at least a polyol or a glycol ether having 8 to 11 carbon atoms, an anionic or nonionic surfactant, a water-soluble organic solvent and water, wherein the ink has a viscosity of 5 mPa·s or more as measured at 25° C., is excellent in ejection stability, storage stability, and is capable of obtaining printed images having good quality and fastness even upon high-speed printing on plain papers.
However, the above conventional water-based inks are still unsatisfactory in optical density upon one-pass printing on plain papers.